The present invention relates to a method for controlling gas pressure filled in a coaxial communication cable in order to minimize variations in phase characteristics of the cable due to variations in ambient temperature.
Recently, observation systems have been employed in which a large antenna is formed by connecting with coaxial cables a number of antenna elements which are spaced apart from one another on the ground. Such observation systems have been employed in specific fields such as radio astronomy in order to intercept radio waves from space. In such systems, reception of signals is carried out for a long observation period such as over several months therefore making it a requirement that the electrical length of the system be constant. In order to meet this requirement, a technique for maintaining the entire system at a constant temperature or a technique for providing a phase control device to compensate for variations of the electrical length of the system due to variations of the ambient temperature in the system has been employed. However, such techniques are disadvantageous in that they are expensive to implement and it is difficult with these techniques to satisfactorily control the ambient temperature.
In order to overcome the above-described difficulty, a technique has been proposed in the art, in which the structure of a cable is modified to considerably reduce the dependence of the electrical length of the cable on temperature. This technique has been applied to cables in which the inner and outer conductors are spaced apart from each other so as to provide electrical insulation therebetween. It is essential that the insulation space between the conductors be constantly maintained in a dry state. For this purpose, dry gas is sealed in the insulation space. Moreover, the sealed gas pressure should be precisely controlled at a constant value and the gas tightness of the connecting terminals at both ends of the cable maintained because, if the sealed gas pressure varies, the electrical length of the cable will undesirably vary. Typically, the electrical length variation factor due to dry air sealing pressure variations is of the order of 10.sup.-4 kg/cm.sup.2. Yet further, the sealed gas pressure is also affected by temperature variations. Therefore, in order to carry out the control operation satisfactorily, it is necessary to determine whether a pressure variation is caused by gas leakage or temperature variation.
The electrical length of the cable is affected by the sealed gas not because of the gas pressure directly but because of variations in the dielectric constant of the gas due to variations in density of the gas. If the sealed gas pressure were to be varied in accordance with temperature variations under the condition that no gas leaks from the cable, the electrical length variational factor of the cable would be quite low, of the order of 10.sup.-8 /.degree.C. for a gas pressure variational factor of 5-6 g/cm.sup.2 /.degree.C. Thus, in order to maintain stable the electrical length of a cable, variations in density of the sealed gas should be minimized.